The present invention relates to an objective lens for an optical disk that focuses a light beam from a light source on an information recording surface of the optical disk such as a digital video disk, a digital audio disk or an optical memory disk for a computer, and an optical head device using the same.
In optical head devices for optical disks, a single lens having an aspherical surface commonly is used as an objective lens for recording information or reproducing recorded information by focusing a light beam onto a diffraction-limited spot on an information recording surface of the optical disk.
In the following, a conventional optical head device will be described, with reference to an accompanying drawing.
FIG. 7 schematically shows a configuration of the conventional optical head device. As shown in FIG. 7, a light beam emitted from a semiconductor laser 151 is subjected to a change in direction of its optical path by a beam splitter 152, and turned into substantially parallel light by a collimator lens 153. The direction of the optical path of this light beam further is changed by a mirror 154 for bending the optical path, then this light beam is focused on an information recording surface 157 of an optical disk 156 by an objective lens 155. This objective lens 155 is driven by an actuator 160. The light beam that has been focused on the information recording surface 157 of the optical disk 156 is diffracted by roughness formed on the information recording surface 157. The light beam that has been reflected and diffracted by the information recording surface 157 of the optical disk 156 passes through the objective lens 155, is subjected to the change in the direction of the optical path by the mirror 154 for bending the optical path, passes through the collimator lens 153, the beam splitter 152 and a cylindrical lens 158, and is focused on a photo detector 159. Based on an electric signal of the photo detector 159, the change in light quantity that has been modulated by the information recording surface 157 of the optical disk 156 is detected, thereby reading data on the information recording surface 157.
In the objective lens 155, a wavefront aberration may be generated because of a manufacturing error during manufacturing. This wavefront aberration theoretically can be divided into aberration components of a third-order spherical aberration, a third-order coma aberration, a third-order astigmatism and a high-order aberration.
Among these aberration components, the third-order coma aberration can be avoided by designed by forming a lens surface of the objective lens 155 to have a rotationally symmetric aspherical surface. However, in reality, decentration (displacement amount in a direction perpendicular to an optical axis) between a first surface 161 of the objective lens 155 on a parallel beam side and a second surface 162 thereof on a focusing side and tilt (inclination with respect to a surface perpendicular to the optical axis) of the first surface 161 of the objective lens 155, the second surface 162 thereof, or both surfaces are generated during manufacture and serve as predominant factors in generating the third-order coma aberration. Out of these two factors, the third-order coma aberration that is caused by the tilt is proportional to substantially the third power of NA (numerical aperture) of the objective lens 155.
In general, when a lens is inclined with respect to an optical axis, the third-order coma aberration is generated. Thus, by adjusting the angle of inclination, the generated third-order coma aberration can cancel out the third-order coma aberration due to the manufacturing error. For this purpose, the objective lens 155 of the optical head device is inclined during an assembly process of the optical head device, so as to reduce the third-order coma aberration. Such an operation is called xe2x80x9ca tilt adjustment,xe2x80x9d and this angle of inclination is called xe2x80x9ca tilt adjustment angle.xe2x80x9d
In recent years, an increasingly higher recording density has been achieved as seen in DVDs (digital video disks) or DVD-RAMs, for example. The key to achieving the densification is how small a spot can be formed on an optical disk, and it is known that the spot diameter can be reduced by increasing the numerical aperture of the lens. Accordingly, in order to achieve higher density, the numerical aperture of the lens gradually has become larger and larger, and now is 0.6 in DVDs. For a further densification, the numerical aperture has to be 0.6 or larger. Considering a manufacturing error during the manufacture of the lens or an assembly error during assembling on the optical head, lenses are designed conventionally so that the generation of decentration of the first surface and the second surface of the lens does not increase aberration and that the incidence of abaxial light does not increase aberration. In spite of a great demand, lenses with a larger numerical aperture have not been commercialized yet partly because, though it is easy to reduce only axial aberration in a lens design, it is very difficult to design a single lens having a sufficient tolerance while taking a manufacturing tolerance and an assembly tolerance into account.
Also, when the numerical aperture is raised, the third-order coma aberration that is caused by the inclination of the disk becomes larger. At present, there is a possibility that warping of the disk causes the inclination of about 0.5xc2x0, which generates the third-order coma aberration of about 70 mxcex in an optical system with a numerical aperture of 0.6 and a focal length of 3.0 mm. In order that a spot is sufficiently small and reproducible in a DVD system, the third-order coma aberration is required to be not larger than 70 mxcex. Thus, when considering the above-described third-order coma aberration generated by the inclination of the disk and the third-order coma aberration generated by the manufacturing error and the assembly error, it is impossible to record and reproduce data in this system.
When the lens having a numerical aperture of larger than 0.6 is designed by the design technique as described above, a satisfactory design is not possible because the aberration due to the decentration and the aberration due to the abaxial light generally are in an inversely proportional relationship. In other words, the lens becomes very difficult to manufacture or assemble. Also, when an optical disk is inclined with respect to an optical axis due to the warp of the disk or the like, a great deal of the third-order coma aberration is generated, so that it becomes impossible to record or reproduce data in this system.
The present invention is made in order to solve the conventional problems described above, and it is an object of the present invention to propose a new design concept for achieving a lens with a large numerical aperture and to provide an objective lens for an optical disk that has a large numerical aperture and is easy to manufacture and assemble, and an optical head device. It also is an object of the present invention to provide an objective lens for an optical disk and an optical head device that can compensate for performance deterioration due to an inclination of the disk.
In order to achieve the above-mentioned objects, the present invention has the following configuration.
An objective lens for an optical disk according to a first structure of the present invention focuses a light beam from a light source. A third-order coma aberration generated when the objective lens is inclined at a unit angle (for example, 0.5xc2x0) is larger than a third-order coma aberration generated when the optical disk is inclined at the same unit angle (for example, 0.5xc2x0). The objective lens is mounted on an actuator for inclining the objective lens according to an inclination amount of the optical disk.
With the first structure, the third-order coma aberration generated by the inclination of the optical disk can be corrected only by inclining the objective lens slightly. Also, since the inclination amount of the objective lens for this correction is small, the amount of astigmatism generated according to the inclination amount of the objective lens also is small. Thus, an excellent residual aberration after the correction is achieved. In addition, since the objective lens is mounted on the actuator and inclined according to the inclination amount of the optical disk, it is possible to provide an optical system that can suppress the generation of aberration due to a warp of the optical disk even when the warp is present.
More specifically, it is preferable that DC/LC greater than xe2x88x920.9 is satisfied, where DC (mxcex) represents a third-order coma aberration generated when the optical disk is inclined at 0.5xc2x0 and LC (mxcex) represents a third-order coma aberration generated when the objective lens is inclined at 0.5xc2x0. Furthermore, it is preferable that DC/LC greater than xe2x88x920.8 is satisfied.
Also, in the first objective lens described above, it is preferable that a numerical aperture is equal to or larger than 0.62. With this structure, a spot diameter of the light beam on the optical disk can be made smaller, achieving a high density recording of the optical disk.
Also, in the first objective lens described above, it is preferable that DC+LC is at least 10 mxcex and not larger than 40 mxcexn. The DC+LC of at least 10 mxcex allows the third-order coma aberration generated by the inclination of the optical disk to be corrected only by inclining the objective lens slightly. Also, since the inclination amount of the objective lens for this correction is small, the amount of astigmatism generated according to the inclination amount of the objective lens also is small. Thus, an excellent residual aberration after the correction is achieved. Furthermore, the DC+LC of not larger than 40 mxcex can suppress a deterioration of aberration characteristics when, because of vibration or the like, the objective lens is inclined unexpectedly. According to the above structure, excellent recording and/or reproducing become possible.
An objective lens for an optical disk according to a second structure of the present invention focuses a light beam from a light source and is used for a recordable and/or reproducible optical disk having a thickness (more precisely, a substrate thickness) of about 0.6 mm (more preferably 0.59 to 0.61 mm). A numerical aperture is at least 0.62 and not larger than 0.66. DC+LC is at least 10 mxcex, where DC (mxcex) represents a third-order coma aberration generated when the optical disk is inclined at 0.5xc2x0 and LC (mxcex) represents a third-order coma aberration generated when the objective lens is inclined at 0.5xc2x0. The objective lens is mounted on an actuator for inclining the objective lens according to an inclination amount of the optical disk. The objective lens is used in an optical system in which the objective lens is inclined so as to generate the third-order coma aberration, which cancels out the third-order coma aberration generated when the optical disk is inclined. A coefficient a is at least 30 and not larger than 35 when a relationship between X and Y is approximated by Y=aX2+bX+c, where X (xc2x0) represents an angle of inclination of the optical disk and Y (mxcex) represents an astigmatism that remains after canceling out the third-order coma aberration generated when the optical disk is inclined at X (xc2x0) by inclining the objective lens.
With the second structure, in recording and/or reproducing in a current DVD system, the third-order coma aberration generated by the inclination of the optical disk can be corrected only by inclining the objective lens slightly. Also, since the inclination amount of the objective lens for this correction is small, the amount of astigmatism generated according to the inclination amount of the objective lens also is small. Thus, an excellent residual aberration after the correction is achieved. In addition, since the objective lens is mounted on the actuator and inclined according to the inclination amount of the optical disk, it is possible to provide an optical system that can suppress the generation of aberration due to a warp of the optical disk even when the warp is present. Also, it is possible to suppress a deterioration of aberration characteristics when, because of vibration or the like, the objective lens is inclined unexpectedly. According to the above structure, excellent recording and/or reproducing become possible.
Next, an optical head device according to a first configuration of the present invention includes a light source, a collimating optical system for converting a light beam emitted from the light source into a substantially parallel light beam, an objective lens for focusing the substantially parallel light beam on an information recording surface of an optical disk, a light beam splitting member for splitting the light beam that has been modulated by the information recording surface of the optical disk, and a photo receiving member for receiving the light beam that has been modulated by the information recording surface of the optical disk. The objective lens is the first or the second objective lens described above.
With the first optical head device described above, the third-order coma aberration generated by the inclination of the optical disk can be corrected by only inclining the objective lens slightly. Also, since the inclination amount of the objective lens for this correction is small, the amount of astigmatism generated according to the inclination amount of the objective lens also is small. Thus, an excellent residual aberration after the correction is achieved. In addition, since the objective lens is mounted on the actuator and inclined according to the inclination amount of the optical disk, it is possible to suppress the generation of aberration due to a warp of the optical disk even when the warp is present. Accordingly, excellent recording and/or reproducing become possible.
In the first optical head device described above, it is preferable that the objective lens is subjected to a tilt adjustment so as to reduce a third-order coma aberration. With this configuration, since it is possible to reduce the third-order coma aberration due to errors during manufacturing lenses and assembling optical head devices by the tilt adjustment, the allowable extent of manufacturing error and assembly error is relaxed, thereby obtaining an inexpensive optical head device.
Next, an optical head device according to a second configuration of the present invention includes a light source, a collimating optical system for converting a light beam emitted from the light source into a substantially parallel light beam, an objective lens for focusing the substantially parallel light beam on an information recording surface of an optical disk, a light beam splitting member for splitting the light beam that has been modulated by the information recording surface of the optical disk, and a photo receiving member for receiving the light beam that has been modulated by the information recording surface of the optical disk. The objective lens has been subjected to a tilt adjustment so as to reduce a third-order coma aberration. A third-order coma aberration generated when the objective lens is inclined at a unit angle (for example, 0.5xc2x0) is larger than a third-order coma aberration generated when the optical disk is inclined at the unit angle (for example, 0.5xc2x0).
Furthermore, an optical head device according to a third configuration of the present invention includes a light source, a collimating optical system for converting a light beam emitted from the light source into a substantially parallel light beam, an objective lens for focusing the substantially parallel light beam on an information recording surface of an optical disk, a light beam splitting member for splitting the light beam that has been modulated by the information recording surface of the optical disk, and a photo receiving member for receiving the light beam that has been modulated by the information recording surface of the optical disk. The objective lens is subjected to a tilt adjustment so as to reduce a third-order coma aberration. DC/LC greater than xe2x88x920.9 is satisfied, where DC (mxcex) represents a third-order coma aberration generated when the optical disk is inclined at 0.5xc2x0 and LC (mxcex) represents a third-order coma aberration generated when the objective lens is inclined at 0.5xc2x0. Furthermore, it is preferable that DC/LC greater than xe2x88x920.8 is satisfied.
Moreover, an optical head device according to a fourth configuration of the present invention includes a light source, a collimating optical system for converting a light beam emitted from the light source into a substantially parallel light beam, an objective lens for focusing the substantially parallel light beam on an information recording surface of an optical disk having a thickness (more precisely, a substrate thickness) of about 0.6 mm (more preferably 0.59 to 0.61 mm), a light beam splitting member for splitting the light beam that has been modulated by the information recording surface of the optical disk, and a photo receiving member for receiving the light beam that has been modulated by the information recording surface of the optical disk. The objective lens has a numerical aperture of at least 0.62 and not larger than 0.66 and has been subjected to a tilt adjustment so as to reduce a third-order coma aberration. DC+LC is at least 10 mxcex, where DC (mxcex) represents a third-order coma aberration generated when the optical disk is inclined at 0.5xc2x0 and LC (mxcex) represents a third-order coma aberration generated when the objective lens is inclined at 0.5xc2x0. A coefficient a is at least 30 and not larger than 35 when a relationship between X and Y is approximated by Y=aX2+bX+c, where X (xc2x0) represents an angle of inclination of the optical disk and Y (mxcex) represents an astigmatism that remains after canceling out the third-order coma aberration generated when the optical disk is inclined at X (xc2x0) by inclining the objective lens.
With the second to fourth optical head devices described above, since the objective lens has been subjected to the tilt adjustment so as to reduce the third-order coma aberration, it is possible to reduce the third-order coma aberration due to errors during manufacturing lenses and assembling optical head devices. Thus, an allowable extent of a manufacturing error and an assembly error is relaxed, thereby obtaining an inexpensive optical head device.
Also, the third-order coma aberration generated by the inclination of the optical disk can be corrected by only inclining the objective lens slightly. Moreover, since the inclination amount of the objective lens for this correction is small, the amount of astigmatism generated according to the inclination amount of the objective lens also is small. Thus, an excellent residual aberration after the correction is achieved, and excellent recording and/or reproducing become possible.
In addition to the above, according to the fourth optical head device, it is possible to suppress a deterioration of aberration characteristics when, because of vibration or the like, the objective lens is inclined unexpectedly. Accordingly, excellent recording and/or reproducing become possible in a current DVD system.
Next, a method for manufacturing an optical head device according to the present invention is a method for manufacturing an optical head device including a light source, a collimating optical system for converting a light beam emitted from the light source into a substantially parallel light beam, the first or the second objective lens for focusing the substantially parallel light beam on an information recording surface of an optical disk, a light beam splitting member for splitting the light beam that has been modulated by the information recording surface of the optical disk, and a photo receiving member for receiving the light beam that has been modulated by the information recording surface of the optical disk. After the light source and the collimating optical system are placed, the objective lens is disposed while being inclined such that a coma aberration of a wavefront of the light beam from the objective lens decreases (in other words, while being subjected to the tilt adjustment).
With the above configuration, since the objective lens is disposed while being subjected to the tilt adjustment, it is possible to reduce the third-order coma aberration due to errors during manufacturing lenses and assembling optical head devices. Thus, the allowable extent of manufacturing error and assembly error is relaxed, thereby obtaining an inexpensive optical head device.
Also, by using the first or the second objective lens, the third-order coma aberration generated by the inclination of the optical disk can be corrected by only inclining the objective lens slightly. Also, since the inclination amount of the objective lens for this correction is small, the amount of astigmatism generated according to the inclination amount of the objective lens also is small. Thus, an excellent residual aberration after the correction is achieved. Moreover, since the objective lens is mounted on the actuator and inclined according to the inclination amount of the optical disk, it is possible to provide the optical system that can suppress the generation of aberration due to a warp of the optical disk even when the warp is present.
As described above, it is possible to obtain an inexpensive optical head device in which excellent recording and/or reproducing are possible.